I always see in popular culture that illiterate people signed contracts with a simple 'x'. However, this has spanned many time periods in popular culture (as many things do), and I was wondering when the practice may have started (if it ever did), and if the mark was legally binding.
How would one be identified on the contract? Would the contract be written out for each individual and an 'x' marked by that person to indicate agreement? How easily could forgeries be made? Am I thinking too deep into contract law concepts through history?
Yes. In fact, it is now!
2 sections of the Uniform Commercial Code (adopted MOSTLY uniformly across the united states, with the exception that some state legislatures have changed some of the the structure in order to fit with their respective statutory scheme) lead to this conclusion.
First, s. 3-401, helpfully titled "SIGNATURE." "(b)A signature may be made (i) manually or by means of a device or machine, and (ii) by the use of any name, including a trade or assumed name, or by a word, mark, or symbol executed or adopted by a person with present intention to authenticate a writing." UCC s. 3-401(b).
Second s. 1-201 General Definitions: "(37) 'Signed' includes using any symbol executed or adopted with present intention to adopt or accept a writing." UCC s. 1-201(37).
I think, looking at those definitions, it's clear that any symbol, even just a simple "X," is acceptable under the Uniform Commercial Code (and therefore acceptable in the present-day United States) for a signature, as long as the intent of the person writing the "X" is that it becomes his/her signature.
Source: The Uniform Commercial Code, which is available in many places, but I accessed through Cornell's Legal Information Institute for my citations in this response: www.law.cornell.edu/ucc
Edit: I only read the title and not the full question. I don't know enough about legal history to know if any of those disputes arose in any noteworthy way, although my brief survey of the case law in my home state hasn't found anything of use. However, I'm gonna leave the rest of the response up as it responds to the title, at least.
FYI, as a nurse, if I have a patient sign with an 'x' and there are two witnesses it is a legal signature. I know your question got answered but I thought I could provide a practical instance of the use of 'x' as a signature.
One relatively modern example of this occurring was at the signing of the founding document of New Zealand, the Treaty of Waitangi. At the town of Waitangi on 6 February 1840, 240 Māori chiefs signed the document. Their names were recorded and they signed next to their name.
A proportion of these signatures were a simple 'X' (a copy of the original document may be viewed here: http://www.nzhistory.net.nz/media/interactive/waitangi-treaty-copy). In this case I would say that the mark was legally binding.
In the ninteenth century American whaling industry, the contract between a sailor and the ship's owner was invariably titled a "Whaleman's Shipping Paper." Each sailor would sign his acceptance of the terms on pre-printed lines at the bottom of the document. But, of course, a substantial percentage of them were illiterate. (I'm using a weasel word like "substantial" because the percentage varied over time. As one might suspect, literacy rates gradually climbed. In general, between 50% and 90% signed their names.) For an illiterate sailor, the ship's agent would write the man's name, and then at the end of the line write "His mark:" and draw a circle, and the sailor would take the pen and make an "X" or a "+" inside the circle.
As a side note, I initially considered using this as a measure of illiteracy among whaling crew, until I learned that some people who could not read had memorized how to sign their own names, so some of the signatures would have been from men that were nonetheless functionally illiterate.
As a followup, were pictographs used to sign documents? If so, when did it shift from these to signatures?